The purpose of these studies is to examine the role of the primary somatosensory cortex (SI) in nociception. We examined the respond of nociceptive neurons in SI to small temperature changes superimposed on noxious levels of thermal stimulation. Two populations of nociceptive neurons were found: one population encoded the intensity of noxious thermal stimulation; the other responded to noxious thermal stimulation, but did not encode the intensity. While major differences in the response of SI nociceptive neurons were found to large temperature changes, no differences were found in the two populations response to small temperature changes. The neuronal discharge of SI nociceptive neurons was correlated with the monkeys' ability to detect noxious thermal stimulation. A significant correlation we found between the peak neuronal discharge and the monkeys' detection speed. In addition, the discharge of SI nociceptive neurons was significantly greater on correctly detected vs non-detected trials. Increases in foreperiod produced an increase both in the monkeys' detection speed as well as an increase in the neuronal discharge of SI nociceptive neurons. These results put to rest the long held view that the cerebral cortex does not participate in the encoding of stimulus features in the noxious range. We conclude that SI cortical nociceptive neurons are involved in the process by which monkeys perceive the intensity of noxious thermal stimulations.